Acrolein is an important starting material for several industrial chemical reactions. The process for producing acrolein involves two major steps: (1) catalytic oxidation of propylene to produce acrolein, and (2) recovery of the acrolein from the reaction products.
Acrolein is typically produced from the vapor-phase oxidation of propylene over a solid-phase catalyst. Conventionally in this reaction, propylene is mixed with air and steam and is passed over the catalyst one time under high temperature and modest pressure, namely a “single-pass” process. Operating conditions used in a single-pass process often achieve a high conversion of propylene, for example, often approaching over 95%. But the operating conditions used in reactors of single-pass processes achieve this high conversion at the expense of reaction selectivity. Moreover, running the reactor at the temperatures employed in single-pass processes typically decreases the life of the catalyst. Use of steam and air in the process also results in significant generation of wastewater and reaction gases that must be purged.
The recovery of acrolein having a high degree of purity from the reaction effluent gas stream is challenging and typically involves several pieces of expensive equipment. Overall, traditional recovery of acrolein involves scrubbing the reaction effluent gas stream with water or a water-solvent mixture in a first separation device to remove acrylic acid, polymeric compounds, and traces of acetic acid. The reaction effluent gas stream is then generally passed to an absorber, where an aqueous solution of acrolein is obtained by absorbing the gas into cold water. The off-gas must be purged as waste gas, because of the large volume of nitrogen contained in the air feed. The aqueous acrolein solution is then typically sent to a desorption column, where the solution is stripped to give crude acrolein. The bottom stream of this column is cooled and reused as an absorbent in the absorber column. The overhead stream from the desorber is distilled (i.e., fractionated) in a light ends column to remove some of the low-boiling byproducts, such as acetaldehyde. The acrolein stream is sent to a heavy ends column where the heavies are removed as the residue stream and the purified acrolein is taken as an overhead stream. This purified acrolein may be used in other industrial processes.
What is needed is a process for producing acrolein in a more efficient and cost effective manner with generation of fewer waste products. In particular, what is needed is a process having improved selectivity, improved overall yield, and improved catalyst life, while allowing isolation of pure acrolein having low levels of impurities without the need for all of the equipment used in traditional acrolein recovery.